Trawl nets and bottom seines possess an initial selectiveness owing to their particular design and mode of operation. A mesh size limitation would vary considerably in effect from one type of trawl net to another. It is important therefore to consider gear selectivity and performance before we begin to think of mesh sizes and their effect. Take a herring vinge trawl for example and two other bottom other trawls like the granton trawl and the shrimp trawl. All use otter boards, all are on the sea bed throughout the duration of the tow and all have the same basic structure of wings, square, bellies, bag and cod end. But their performances differ as much as those of a racing car, a truck and a tractor. All three nets could be fishing in the same area, yet the vinge trawl might take only herring, the granton trawl only demersal fish and the shrimp trawl primarily shrimp or prawn. Mesh size has little to do with this species selection. It is a function of other aspects of gear performance - speed of tow, headline height, ground contact, flow of water through the body of the net, otter door spread, length of ground-cables, and so on. The vinge trawl would have the smallest mesh in the cod end, yet it would take no cod, haddock or shrimp. The shrimp or prawn trawl would have a smaller mesh than the granton trawl yet it would capture only a small proportion of the bigger fish the granton trawl would take. To imagine that in these cases the size of mesh controls the size of fish caught would be as simplistic as to think that the respective speeds of the racing car, truck and tractor were determined by the size of their wheels. In contrast to the trawls designed to capture one species or group of species, there are the multi-species trawls or combination trawls which take a great variety of fish. The North Sea prawn and fish trawl is a combination net designed to capture Nephrops norvegicus or Norway prawns, plus demersal fish like cod, haddock, skate, monks, plaice and lemon sole. Most bottom trawls in the tropics and sub-tropics are multi-species trawls taking fish which vary greatly from each other in size and shape. There is no common selectivity factor and no common minimum size or length for the various species. For these fisheries, a mesh regulation is at best an inadequate means of reducing juvenile mortality. It can be aimed only at the smallest of the main commercial species.

1. PROBLEMS ON FISHING GEAR
SELECTIVITY
Ashish sahu

2. INTRODUCTION
 Selectivity is an important tool for effective
management of fisheries.
 It is defined as the ability to target and capture fish
by species, size or sex or combination of these
during harvesting operation, allowing all the
incidental by- catch to be released unharmed.
 By regulating the minimum mesh size of a fishing
fleet ,the minimum landing size of the target
species can be determined.
 Several researchers stress the importance of
selecting the optimum mesh size from the
standpoint of conservation of population.

3.  Selectivity is not a single process.
 The act of catching a fish involves several stages.
The fish must encounter the net, be caught and
finally be retained.
 Any one of these stages can be selective.
 Selectivity is expressed as a ratio referred to as
selectivity coefficient.
 Fish population exposed to the gear can be
estimated through the use of non-selective gear,
which catches all the fish that comes in contact.
Fish caught in the fishing gear
Selectivity coefficient =
Fish population expose to the gear

4. ADVANTAGES
 Regulate mesh size and minimum landing size.
 Allow gear technologist to design gear which permits
escapement of unwanted by-catch.
 Help in return of juveniles back into the system-help in
continued reproduction and growth of the stocks.
 Reduction of destructive fishing practices.
 Allow harvest of targeted fish thereby decreasing
labour for sorting.

5. SELECTIVITY CURVES
 Selectivity is represented by a curve fitted to points
representing the percentage of fish either retained
by the gear or escaping from it at each interval
against the fish length.
 Selection curves are often plotted against the ratio of
the fish girth/mesh perimeter or fish length.
 Normally two common types of selection curves or
ogives are formed depending on the type of gear and
type of selection.

6. NORMAL CURVE

7. S(L)=exp(-(L-Lm)²/2S²)
Where:-
S(L) : length based gear selectivity
L : length interval midpoint
Lm : optimum length for being caught
S : standard deviation of normal curve

8.  Passive gear like gill nets generally have normal
curves also known as bell shaped or symmetrical
curves .
 Width of the curve represents the selection range
and the height corresponds to optimum size of fish
caught by the gear.
 In symmetrical curves, the ability to catch fish
decreases equally on both sides.

9. SIGMOID CURVE

11.  It is also termed as asymmetrical curve or ‘S’
shaped curve.
 The gear prevents escapement of larger ones but
allow smaller ones to escape in relation to size i.e.,
it catches increasingly more fish as the size
increases.
 Selectivity of gear like trawl is generally
represented by sigmoid curve.

12. GEAR SELECTIVITY

13. 1. GILL NET
 Gill nets are vertical panels of netting normally set
in straight line.
 Catching process in gill nets can be either by
wedging, gilling or entangling.
 Capture by gilling, and wedging are dependent on
the shape of species of the fish encountered.
 Gillnets are size and species selective.
 Tangling may be less dependent on mesh size .

14. METHOD OF CAPTURE
 Gillnet selection (Karlsen and Biarnason, 1986)

16. FACTORS INFLUENCING THE SELECTIVITY OF
GILL NETS
1. Mesh size:
 Mesh size has been the greatest influence on
selection process in gill nets.
 Selection occurs at both ends of the selection range
as smaller fishes pass through the meshes without
being caught and larger fishes will not be able to
penetrate the meshes.
 There is an optimum size at which a fish is most
likely to be held ,unable to escape.

17. 2. Netting :
• The important characteristics of a gill net that affect
selectivity are its visibility, stretchability of meshes
,construction ,thickness and colour of the material.
• The elasticity and flexibility affect selectivity.
• Meshes of a more elastic twine can be stretched to
a larger size by a struggling fish and increased
elasticity results in the capture of a larger average
size of fish and a wide selection range.
• As the twine is made thinner, stretchability and
flexibility increases.

18. 3. Gear design :
• Gear construction ,hanging coefficient and rigging
affect net behaviour in the water which in turn affect
both the selectivity and efficiency of fishing.
• Loosely hung nets allow more fish to become
entangled.
• The headline and footropes and their relative
magnitudes are important in ensuring that the net
shape and fishing power are maintained irrespective
of tide and current conditions.

19. 4.Operational factors :
• Location , depth of fishing and orientation of fishing
gear affect the efficiency and hence the selectivity
of the gear.

20. MEAUREMENT OF SELECTIVITY
1.Direct estimate
 fishing a known population
no of fish population in the sample is known
(catching a tagged fish)
slectivity=no of fish caught/(no of fishes in the
population*fishing effort)
 Comparison with a gear of known selectivity
compare the catch of experimental gear with the
population estimate from the net of known
selectivity

21. 2.Indirect method:
 Compare size distribution of catches in the gillnet of
different mesh size
 2 method
 Using type B curves as intermediaries.
Type B curve (selectivity of different mesh size to
one size class of fish) is plotted first & from this
plot type A curve (selectivity of one mesh size to
different size of fish) is estimated.
lets Length frequency distribution of catch
obtained in 2 gill net of mesh size a & b ,Lo is the
optimum length of fish
Then coefficient K=2a×b/Lo(a+b)

22.  Fitting a predetermined distribution
 Type A curve is estimated algebraically from catch data
with out plotting intermediate type B curve.
 Two nets are simultaneously operated in same area
 Holt (1963) introduced this method
Y=ln(Cb/Ca)= a+bL
Selection factor= -2a/(b(m1+m2))
a&b are regression constant
M1&m2 mesh size
Ca catch from net a
Cb catch from net b
L is midpoint of length class
Probability of capture P= exp(-(L-Lm)²/2S²)

23. ESTIMATING GILLNET SELECTION FROM
GIRTH MEASUREMENT
 Max girth= k*mesh perimeter
 k is a constant and value near1.25 with a range
of 1.08-1.35

25. 2.TRAWLS
 Trawling is a method of fishing that involves pulling a
fish net through the water behind one or more boats.
 The net that is used for trawling is called trawl.
 Bottom trawling is towing the trawl along the sea
floor.
 Mid water trawling is towing the trawl through free
water above the bottom of the ocean or benthic zone.

27. FACTORS INFLUENCING SELECTIVITY OF
TRAWLS
1.Mesh size:
• Size selectivity is controlled by the mesh size of the “cod-
end” ,the part where fish is retained.
• There are number of “fixes”, such as tying a rope around the
“cod-end” to prevent the mesh from opening fully .
• By catch reduction grids or square mesh panels of net can
be fitted to parts of trawl, allowing certain species to escape
while retaining others.
• By catch reduction devices other than square mesh cod
ends, like turtle excluder devices , juvenile fish excluder
cum shrimp sorting devices ,big eye BRD are also used in
trawls.

28. 2.Gear design:
• Trawl nets selectivity also depend upon their particular
design and mode of operation.
• Species caught may vary between different types of
trawls.
• Selectivity is a function of other aspects of gear
performance such as towing speed, headline height ,
ground contact ,flow of water through the net, otter
door spread ,length of ground cables etc.

29. 3. Area swept :
• Sea bed swept by the gear is important for its
effectiveness.
• Ropes ,wires, cables and bridles are used to herd fish
in the direction of the net.
• The species that do not respond to herding by ropes
include shrimps, prawns ,clupeids ,mackerels etc.
• Trawl net for clupeids and mackerels have very large
net mouth opening where as for shrimps have a wide
but low net mouth and short cables or bridles.

30. MEASUREMENT OF SELECTIVITY

33. 3. POLE AND LINE
 Pole and line fishing usually means a particular type of
rod and line fishing in which fish are attracted to the
surface with bait fish ,in a process called chumming.
 Target species : Skipjack tuna, Yellowfin tuna, Bigeye
tuna, Atlanic bluefin tuna.
 Pole and line fishing have low levels of by- catch
relative to other major fishing methods.
 It is much more selective ,other creatures are rarely
caught and if they are, they can usually be returned to
water unharmed.
 Survival chance of released by-catch are high due to
use of barbless hooks and the quick release from
them.

35. 4.SQUID JIGS
 Jigging is the practice of fishing with a jig, a type of fishing
lure.
 A jig consists of a lead sinker with a hook molded into it and
usually covered by a soft body to attract squid.
 It is important to keep the jigs moving constantly in the
water.
 This is usually done by jerking the line , quickly pulling in
slack, jerking once again and so on , until the jig is back to
the surface.
 Squid jigging most often takes place at night with bright
overhead lights to attract the squid.
 Jigs of various types, and colour are attached to the
handline at 70 to 90cm intervals.
 The lines are lowered to 30 to 100m depth depending on
the strength of the lights used.

37. TRAPS
 Traps in which the fishes are retained or entered
voluntarily and will be hampered from escaping.
 Traps are baited or not.
 Target species
• large stationary nets or barrage are used to catch
migrating fishes.
• pots are used for catching lobsters , crabs, octopus
and eels.
• Aerial traps are used for catching fishes like
mullets, which jump out of water on disturbance.

39. MEASUREMENT OF SELECTIVITY

40. CONCLUSION
 Selective fishing enables a fishing operation to avoid
catching non-target species or limiting species in
mixed fishery.
 Selective fishing methods should be evaluated in
terms of broader environmental effects, and should be
viewed as a part of the toolbox for sustainable
management of marine ecosystems.

41. REFERENCES
 www.fao.org.docrep
 www.ices.dk.blogs
 www.unuftp.is.stock-assessment
 https://www.sciencedirect.com
 https://www.researchgate.net
 www.seafish.org
 Sally N Thomas gillnet selectivity, Central Institute
Of Fishing Technology

42. THANK YOU….